Floating platform for offshore drilling or production of hydrocarbons

ABSTRACT

A floating platform for offshore drilling or production of hydrocarbons comprises a topsides with drilling and/or production equipment ( 8 ), and a substructure comprising a lower pontoon ( 3 ) and columns ( 4 ) connecting the pontoon ( 3 ) to the topsides ( 1 ). During its operation the platform is exposed to wave forces ( 9 ) which cause heave motion (v) and roll and pitch motion (p) of the platform. The substructure has a draught ( 11 ) which is at least 40 metres, preferably at least 50 metres and most preferred at least 60 metres, and the ratio between the distance ( 16 ) between the columns&#39; centre axes ( 25 ), measured along a side of the substructure, and the lower substructure&#39;s draught ( 11 ) is between 1.0 and 1.5, preferably between 1.2 and 1.4 and most preferred between 1.3 and 1.35.

[0001] The invention relates to a floating platform for offshoredrilling or production of hydrocarbons, comprising a topsides withdrilling and/or production equipment, and substructure comprising alower pontoon and columns connecting the pontoon to the topsides, whereduring its operation the platform is exposed to wave forces which causeheave motion and roll and pitch motion of the platform.

[0002] On account of wave action, floating structures of all types willexperience motion in the water. Waves in the sea are a highly complexphenomenon, and the structure is exposed to an excitation from waves indifferent directions and with different oscillating periods. Thefloating structure partly acquires a drifting motion, i.e. a movement ofthe structure, and partly an oscillating motion. The oscillating motioncan be divided into resiprocating linear motion along three axes, i.e.the two horizontal directions and the vertical direction, andreciprocating rotating motion about the same three axes, thus giving atotal of six independent motion components.

[0003] For a floating platform it is normally three of the six motioncomponents which are of the greatest importance, viz. upwardly anddownwardly-directed vertical motion, usually called heave, reciprocatingrotating motion about a horizontal longitudinal axis, usually calledroll, and recprocating rotating motion about a horizontal transversalaxis, usually called pitch.

[0004] When engineering a floating platform it is desirable to designthe platform in such a manner that the platform has the least possibleoscillating motions. This applies in particular to floating platformswhich have dry well completion on the main deck. In addition, the designof the platform must also take into consideration space requirements,load capacity requirements, possibly requirements regarding storagecapacity for hydrocarbons, and also other characteristics which aredesirable for the floating platform.

[0005] It is usually possible to design a floating platform in such afashion that some of its motion components in the water are moderate. Todesign a platform where all the motion components are avoided, however,is no easy task, since designing the platform with a view to avoidingone motion component usually results in an increase in the tendency ofthe platform to acquire another motion component.

[0006] Floating platforms are normally designed in such a manner thatheave, roll and pitch motion partly occur at the waves' excitationperiod, and partly at the platform's natural periods for the respectivemotions. There is usually little wave excitation at the natural periodfor roll and pitch. Nevertheless, roll and pitch could be activated atthe natural period for roll and pitch due to wind and the influence ofheave motion.

[0007] In order to minimise the motion of the floating platform in thewater and satisfy requirements for space and load capacity, two maintypes of floating platforms have been developed. One type is known underthe name SPAR platform, and comprises an elongated, vertical subsea bodyextending deep down into the water. The SPAR platform has little heavemotion, but has relatively substantial roll and pitch motion inanti-phase with corresponding wave motions. The other type of floatingplatform is the column platform where three or more columns connect theplatform's topsides with one or more pontoons. This type of platformnormally has a length/width which is considerably larger than itsdraught, in order to provide a substantial amount of space and loadcapacity. The column platforms have greater heave motion than the SPARplatforms, and have roll and pitch motion in phase with the waves.

[0008] U.S. Pat. No. 4,934,870 describes a floating structure withlimited heave oscillations. An elongated element has a lower end whichis connected to the seabed, and an extendible tension device isconnected between a platform deck and the upper end of the elongatedelement. The tension device comprises devices which exert forces whichcounteract the heave motions.

[0009] Other floating platform structures are described in U.S. Pat. No.3,986,471, U.S. Pat. No. 5,931,602, U.S. Pat. No. 4,913,238, U.S. Pat.No. 5,439,321, U.S. Pat. No. 4,215,950, U.S. Pat. No. 4,793,738, U.S.Pat. No. 4,753,553, U.S. Pat. No. 4,702,321, U.S. Pat. No. 4,194,568 andEP 0 256 177.

[0010] In U.S. Pat. No. 4,829,928 it is described a platform which has anegatively buoyant pontoon suspended from the balance of the platform toincrease heave resonant period to at least 25 seconds. Tendons suspendthe pontoon to a depth where dynamic wave forces do not materially actdirectly on it in seas of normally occurring periods of about 15 secondsbut do in seas of periods above 15 seconds. Columns and an upper pontoonprovide buoyancy for the platform. The platform motion is reduced byincreasing its mass and the damping component for the platform. Onedrawback with this solution, which one would like to avoid, is thecomplications related to the presence of a large suspended pontoon thatcannot be utilised to anything useful and the suspension mechanism.Adding extra structural elements to a platform construction like asuspended pontoon and suspension devises, will add complication in bothconstruction, building, running, maintenance and repairs of theplatform, that one wants to avoid.

[0011] The object of the invention is to provide a floating platformwhich has little motion in the water, while at the same time having asubstantial amount of space and considerable load capacity.

[0012] The object is achieved with a floating platform of the typementioned at the beginning which is characterized by the features whichare indicated in the claims.

[0013] The invention therefore relates to a floating platform foroffshore drilling or production of hydrocarbons, comprising a topsideswith drilling and/or production equipment, and a substructure comprisinga lower pontoon and columns connecting the pontoon to the topsides,where during its operation the platform is exposed to wave forces whichcause heave motion and roll and pitch motion of the platform.Surprisingly, it has been found that a platform with dimensions anddimensional ratios according to the invention encounters little heave,roll and pitch motion. According to the invention the substructure has adraught which is at least 40 metres, preferably at least 50 metres andmost preferred at least 60 metres. Moreover, the ratio of thesubstructure's draught to the distance between the columns' centre axes,measured along a side of the substructure is between 1.0 and 1.5,preferably between 1.2 and 1.4 and most preferred between 1.3 and 1.35.Calculations show that by means of the invention it is possible toprovide a floating platform which has little motion in the water, whileat the same time having a substantial amount of space and considerableload capacity.

[0014] The invention will now be explained in greater detail inconnection with a specific embodiment, and with reference to theaccompanying drawings, in which:

[0015]FIG. 1 illustrates a side section through a floating platformaccording to the invention,

[0016]FIG. 2 illustrates a cross section viewed from above throughcolumns which form part of a floating platform according to theinvention, and

[0017]FIG. 3 is a curve showing oscillating motion as a function ofoscillating period.

[0018]FIG. 1 illustrates a floating platform for offshore drilling orproduction of hydrocarbons, comprising a topsides 1 with drilling and/orproduction equipment 8, and a substructure comprising a lower pontoon 3and columns 4 connecting the pontoon 3 to the topsides 1. The platformis lying in the water 23, with the waterline indicated by referencenumeral 10. The topsides 1 may include one or more decks with equipmentand installations for carrying out a number of functions which arenecessary in connection with a floating platform, for example livingquarters, hoisting cranes and electrical generators. The columns 4 areconnected to the pontoon via transition portions 7. The columns 4, thetransition portions 7 and the pontoon 3 are provided with buoyancy tanks(not shown) and ballast water tanks which can be filled with water inorder to adjust the platform's position in the water 23, and possiblystorage tanks for hydrocarbons.

[0019] The platform may be of a type which is connected to the bottom bymeans of approximately vertical tension legs, it may be connected to thebottom via slanting, slack moorings, or it may be kept almost immobilein the water with dynamic positioning, by means of positioningpropellers controlled by an electronic control system. How the platformis moored or kept immobile is not within the scope of the invention andis not illustrated in the figures.

[0020]FIG. 2 illustrates a cross section viewed from above through thecolumns 4, through the waterline 10. It shows that the pontoon 3 isoctagonal, and in the middle has an octagonal opening 24. It also showsthat the columns 4 are four in number, and that the platform's topsides1 is rectangular. The number of columns and the shape of the pontoon andthe topsides are partly chosen on the basis of sizing criteria and couldhave been different.

[0021] When the floating platform is located in the water it willacquire a motion which is divided into vertical upwardly anddownwardly-directed heave motion, which is indicated in FIG. 1 by thedouble arrow v, and roll and pitch motion, which means a reciprocatingrotation of the platform about horizontal axes. In FIG. 1 the roll andpitch motion is indicated by the double arrow p, and the rotation centrefor the roll and pitch motion is indicated by reference numeral 5. Itcan be seen that the rotation centre 5 is located slightly above theplatform's centre of gravity 6.

[0022] The platform's motion is partly dependent on the design of theplatform, such as the platform's mass, damping and rigidity, and partlythe driving forces, i.e. wind and wave forces.

[0023] The driving forces for the pitch and roll motion have manycontributors. As described in the literature, for example O. M.Faltinsen: Sea Loads on Ships and Offshore Structures, the wave forcescan be divided into mass forces and pressure forces.

[0024] The water particles in a wave will constantly have cyclicaccelerations. A body which is located in a wave is influenced by forceswhich can be calculated on the basis of the water particles'accelerations and displaced liquid volume. These forces are called massforces.

[0025] On account of the waves' upwardly and downwardly-directed motion,there will also be varying liquid pressure in a wave. A body which islocated in a wave will be influenced by forces from this varying liquidpressure, and these forces are called pressure forces.

[0026] The mass forces and the pressure forces are indicated in FIG. 1by arrows with reference numeral 9. The forces 9 from the wavesinfluence the external surfaces of the pontoon 3, the transitionportions 7 and the portions of the columns 4 which are located underwater. The wave motion is a highly complex phenomenon, and the forces 9are continuously changing and acting in different directions. The sum ofall the forces 9 represents the waves' total influence on the platform.The forces give rise to moments which can be divided into the followingcomponents:

[0027] Moments due to mass forces on the columns

[0028] Moments due to pressure forces on the columns

[0029] Moments due to mass forces on the pontoons

[0030] Moments due to pressure forces on the pontoons

[0031] These components act in different directions and with relativemagnitude depending on the oscillating period of the waves, the draughtand principal dimensions of the substructure.

[0032] By means of a spectral analysis of a typical wave motion, whichcan be performed by a computer program of a known type, it is possibleto find the extent of excitation conveyed to the platform by the variouscomponents in the wave motion. In FIG. 3 this is illustrated in a curve30, called the wave excitation spectrum. The wave motion's excitationenergy, which has the unit m²s, is shown here as a function of thewaves' oscillating period t. It is seen that the waves have anoscillating period of between approximately 6 and 25 seconds. The curve30 has a top 31 for an oscillating period t_(t) of approximately 17seconds for the sea state represented by the curve. It can be seen thatthere is little excitation from waves with a oscillating period of over20 seconds. An analysis of typically occurring wave motion in the seahas shown that waves with an oscillating period of over 20 secondsgenerally convey little excitation of oscillations.

[0033]FIG. 3 also illustrates a curve 32 for a platform's heave motionas a function of oscillating period t. The curve 32 is a transferfunction for the heave motion with the unit m/m, and illustrates theplatform's heave motion per wave amplitude of the excitation waves. Itis seen that the curve 32 has a pronounced top 33 for an oscillatingperiod t_(z) of approximately 22 seconds. This oscillating period iscalled the platform's natural period t_(z) for heave motion.

[0034] It has been found that, in order to avoid excitation of heavemotion of the platform, the weight of the topsides 1, the design,displacement, number and position of the columns 4, and the design anddisplacement of the pontoon 3 should be adapted so as to give theplatform a natural period t_(z) for heave motion which is at least 1.1,preferably at least 1.2 and most preferably at least 1.3 times themaximum period for significant oscillation excitation from the waves,i.e. 20 seconds, and therefore the platform's natural period t_(z) forheave motion should be at least 22, preferably at least 24 and mostpreferred at least 26 seconds.

[0035] With the natural periods t_(z) for the platform's heave motionwhich can realistically be expected to be achieved, there will be acertain amount of wave motion. It has been found that the effect of thepressure forces' excitation of heave motion decreases with increasingdraught. With the platform according to the invention, therefore, thesubstructure has a draught 11, i.e. the distance from the waterline 10to the bottom of the pontoon 3, which is at least 40 metres, preferablyat least 50 metres and most preferred at least 60 metres. At thesedepths the vertical influence of the waves is insignificant, thus givingthe platform a small heave motion.

[0036] In order to avoid excitation of roll and pitch motion of theplatform, according to prior art the weight of the topsides 1, thedesign, displacement, number and position of the columns 4 and thedesign and displacement of the pontoon 3 must be adapted in order togive the platform a natural period for roll and pitch motion which isdifferent to the waves' excitation periods.

[0037] It has been found that the mass forces on the pontoon 3 and thecolumns 4 for the most important of the waves' periods act in theopposite direction. It has been found that the mass forces on thepontoon 3 have a greater influence on the platform's motion than themass forces on-the columns 4 when the substructure's draught 11 is smallrelative to the distance 16 between the columns' 4 centre axes 25,measured along a side of the substructure. When the substrucutre'sdraught 1 is substantially greater than the distance 16 between thecolumns' centre axes 25, the mass forces on the columns 4 will make agreater contribution to the platform's motion than the mass forces onthe pontoon 3. With the platform according to the invention, therefore,the ratio of the substructure's draught 11 to the distance 16 betweenthe columns' centre axes 25 is between 1.0 and 1.5, preferably between1.2 and 1.4 and most preferred between 1.3 and 1.35, since it has beenshown that the various components of the wave forces then cancel eachother out, and the resulting moment from the mass forces is thereforevery small, and the waves' excitation of roll and pitch motion issubstantially eliminated.

[0038] It has been found that a platform which both fulfils therequirement that the substructure should have a draught 11 which is atleast 40 metres, preferably at least 50 metres and most preferred atleast 60 metres, and the requirement that the ratio between the distance16 between the columns' centre axes 25 and the substructure's draught 11should be between 1.0 and 1.5, preferably between 1.2 and 1.4 and mostpreferred between 1.3 and 1.35 provides a particularly advantageousfloating platform, where both the heave motion and the pitch and rollmotion are slight.

[0039] It has further been found that an additional reduction of theplatform's motions in the water is achieved when the ratio of the totalcross sectional area of the columns 4 in the waterline 10 to theplatform's total mass is between 0.005 and 0.018 m²/metric ton, andpreferably between 0.08 and 0.015 m²/metric ton, where total crosssectional area of the columns 4 in the waterline 10 means total crosssectional area of the columns 4 as they are illustrated in FIG. 2.

[0040] Calculations show that a floating platform which hasconstructional features according to the invention has little motion inthe water, while being capable of being designed so as to have asubstantial amount of space and considerable load capacity.

1. A floating platform for offshore drilling or production ofhydrocarbons, comprising a topsides with drilling and/or productionequipment (8), and a substructure comprising a lower pontoon (3) andcolumns (4) connecting the pontoon (3) to the topsides (1), where duringits operation the platform is exposed to wave forces (9) which causeheave motion (v) and roll and pitch motion (p) of the platform,characterized in that a) the substructure has a draught (11) which is atleast 40 metres, preferably at least 50 metres and most preferred atleast 60 metres, and b) the ratio of the substructure's draught (11) tothe distance (16) between the columns' centre axes (25), measured alonga side of the substructure is between 1.0 and 1.5, preferably between1.2 and 1.4 and most preferred between 1.3 and 1.35.
 2. A floatingplatform according to claim 1, characterized in that the weight of thetopsides (1), the design, displacement, number and position of thecolumns (4) and the design and displacement of the pontoon (3) areadapted so as to give the platform a natural period (t_(z)) for heavemotion which is at least 22, preferably at least 24 and most preferredat least 26 seconds.
 3. A floating platform according to claim 1 or 2,characterized in that the ratio of total cross sectional area of thecolumns (4) in the waterline (10) to the platform's total mass isbetween 0.005 and 0.018 m²/metric ton, and preferably between 0.08 and0.015 m²/metric ton.